What are rocks?
Have you ever picked up a stone? That stone is a rock. Rocks are not just simple pebbles. They are groups of minerals stuck together.
Minerals have one specific chemical recipe. But a rock can mix many different minerals.
A rock can also contain other materials. Sometimes it holds fragments of older rocks. It can even hold organic matter from living things. All of these pieces naturally cement together. This process forms what we call a rock. These rocks create the Earth’s outer layer, or crust. They give us the raw materials for many important things. We use rocks to build our houses and roads. Rocks also break down to become the soil that grows our food. Natural processes constantly change and shape rocks. These processes form our mountains and canyons. So next time you see a rock, remember it is a key piece of our world.
The Rock Cycle
Rocks are always changing. This change is called the rock cycle. The cycle never stops. It takes millions of years. Heat from inside the Earth powers it. Wind and water on the surface also power it.
First, wind and water break big rocks down. This is called weathering. Then, water or wind carries the small pieces away. This is called erosion. Next, those small pieces settle in a new place. This is called deposition. The pieces pile up and get pressed together. Over time, they stick and form a new, hard rock. We call this new rock a sedimentary rock.
Sometimes, rocks get pushed deep underground. The great heat there melts them into liquid rock. This liquid rock can cool down and become solid. Then it becomes a new kind of rock, an igneous rock.
Other times, rocks get squeezed by great pressure. They also get heated, but not melted. This heat and pressure changes them. It forms a completely different rock. This new rock is called a metamorphic rock.
The rock cycle connects everything on Earth. It makes sure that all rocks change over time. No rock stays the same forever.
First, melted rock cools down. This makes igneous rock.
Next, sand and mud get pressed together. This makes sedimentary rock.
Then, old rocks get heated and squeezed. They change into metamorphic rock.
| Feature / Type of Rock | Igneous Rocks | Metamorphic Rocks | Sedimentary Rocks |
|---|---|---|---|
| Formation Process | Formed by cooling and solidification of molten magma or lava | Formed when existing rocks are changed by heat, pressure, or chemical processes without melting | Formed by deposition, compaction, and cementation of sediments or organic remains |
| Origin Material | Magma or lava | Pre-existing igneous, sedimentary, or metamorphic rocks | Weathered and eroded rock fragments, minerals, or biological material |
| Structure | Crystalline (interlocking crystals) | Foliated (layered) or non-foliated depending on pressure | Often layered (strata) |
| Examples | Granite, Basalt, Diorite | Marble, Slate, Schist, Gneiss | Sandstone, Limestone, Shale |
| Texture | Coarse-grained (intrusive) or fine-grained (extrusive) | Varies: banded, foliated, or massive | Clastic, chemical, or organic |
| Fossils | Rarely contain fossils (heat destroys them) | Usually do not contain fossils (recrystallization destroys them) | Often contain fossils |
| Location | Found in volcanic regions or deep inside Earth | Found in mountain belts where tectonic activity occurs | Found in river beds, oceans, lakes, and deserts |
| Strength/Hardness | Generally hard and strong | Usually harder and more compact than parent rock | Usually softer and more porous |
Igneous Rocks:
Igneous rocks are formed when molten rock (magma or lava) cools down and hardens.
- Intrusive Rocks: These form when magma cools slowly beneath the surface. Because of the slow cooling, you can often see large mineral crystals, like in Granite.
- Extrusive Rocks: These form when lava cools quickly on the surface. They have very small or no crystals, and can look glassy or have a bubbly texture. Basalt is a great example.
Classification of igneous rocks by their composition:
- Felsic: These rocks are high in silica and are typically light in color, like Granite.
- Mafic: These rocks are rich in magnesium and iron, making them dark in color, like Basalt.
Intrusive Rock Structures
When hot magma (molten rock) from inside the Earth moves up, sometimes it does not come out as a volcano. Instead, it stays inside the Earth and cools slowly. While cooling, it makes different shapes inside the rocks. These shapes have special names.
| Name | Shape (Imagine) |
|---|---|
| Laccolith | Like a mushroom – flat at bottom, bulging at top |
| Batholith | A very big rock mass deep inside Earth – like a giant underground mountain of rock |
| Phacolith | Like a lens or banana shape – it fits in the folds of rocks (up and down) |
| Sill | Like a blanket of rock between layers of other rocks |
| Dyke | A wall of rock cutting across other rock layers |
| Lopolith | A large bowl-shaped (concave) mass of igneous rock formed beneath the Earth’s surface |
| Xenolith | A foreign rock fragment trapped inside another rock (carried up by magma and solidified) |
